Lithium metals and lithium alloys are used as typical negative electrode materials in nonaqueous secondary batteries, but, when they are used, the lithium metal grows into a dendritic form during charging and discharging to generate a so-called dendrite which becomes a cause of internal short, or the high activity of the dendrite itself poses a possible danger of causing firing and the like.
On the other hand, calcined carbonaceous materials capable of reversely intercalating and deintercalating lithium have been put into practical use. Such carbonaceous material has a relatively small density which poses a disadvantage of having low capacity per volume. Because of this, use of the carbon material by pressing or laminating lithium foil thereto is described in JP-A-5-151995 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), but it cannot resolve the aforementioned problems.
Also, methods in which oxides of Sn, V, Si, B, Zr and the like or composite oxides thereof are used in negative electrode materials have been proposed (JP-A-5-174818, JP-A-6-60867, JP-A-6-275267, JP-A-6-325765, JP-A-6-338324, EP-615296). It is said that negative electrodes of these oxides or composite oxides provide nonaqueous secondary batteries having a large charging capacity of 3 to 3.6 V class when combined with a positive electrode of a certain type of lithium-containing transition metal compound and have markedly high safety, because they hardly generate dendrite within the practical range. However, batteries in which these materials are used have a serious problem in that their charge and discharge characteristics are not sufficient, and their charge and discharge efficiency in initial cycles is particularly low. That is, it is assumed that a portion of lithium molecules intercalated in the negative electrode during the charging step cause a plurality of irreversible side reactions during several initial stage cycles, so that lithium does not move into the positive electrode side during the discharging step, thereby causing capacity loss due to unnecessarily consumed lithium in the positive electrode. In order to compensate for such a capacity loss, it may be possible to intercalate lithium into the negative electrode material in advance in an amount corresponding to the capacity loss, but sufficient effect has not been obtained yet by such means.
The object of the present invention is to obtain a nonaqueous secondary battery which has 1) high charge and discharge capacities and excellent charge and discharge cycle characteristics and 2) a high energy density.